One-dimensional nanostructures such as nanowires and carbon nanotubes are known to be promising candidates for highly sensitive and miniaturized molecular sensors. In particular, sensors based upon semiconductor nanostructures, such as carbon nanotubes, silicon (Si) nanowires, tin oxide (SnO2) nanowires, and indium oxide (In2O3) nanowires, are particularly promising as their detection is based on the principle of a change in surface charge of the nanostructures in the presence or absence of molecular species.
Due to a high surface-to-volume ratio of nanowires, their electronic conductance may be sensitive enough to the surface species such that single molecule detection becomes possible. However, most of the existing studies based on “bottom-up” nanostructures are limited by complex integration, requiring transfer and positioning of an individual nanostructure, making reliable ohmic contacts and the control of doping concentrations. In addition, delivery of a biological receptor such as enzymes, antibodies, proteins or biological macromolecules that can be used for detection of an analyte remains a challenge in nanostructure sensor devices. The same applies to the delivery of the analyte to be detected. Currently, delivery is normally accomplished by using syringe pumps, micropipettes or atomic force microscopy (AFM) tips (Dip-pen) which is crude, time-consuming and trial and error based, as there is a need to accurately locate the nanowire with precision. Therefore, such delivery methods are only meant for laboratory experiments and are not useful from a product realization point of view.
Several attempts have been made to address these problems so as to enable utilization of nanowires for sensing in a fluidic environment. Amongst them are biosensors which incorporate the capability to deliver various fluids for surface modification, immobilization, detection and testing. One approach is described in United States Patent Application 2002/0117659. This application discloses nanoscale devices having a sample exposure region and a nanowire or functionalized nanowires for detecting the presence or absence of an analyte suspected to be present in a sample, and method for using the same. In United States Patent Application 2002/0117659, the nanowires act as sensors and are capable of a property change when it is in contact with a sample suspected of containing an analyte. According to this patent application, nanowires are fabricated by metal-catalyzed chemical vapor deposition (CVD) and laser catalytic growth.
In United States Patent Application 2002/0117659, the sample exposure region may be any region in close proximity to the nanowire wherein a sample in the sample exposure region addresses at least a portion of the nanowire. An example of the sample exposure region is a fluid flow channel and this may be formed by using a polydimethylsiloxane (PDMS) mold. Channels can be created and applied to a surface, and a mold can be removed. Alternatively, the channels can be made by fabricating a master using photolithography and casting PDMS on the master. The nanowire and the fluidic channels are formed separately by the above-mentioned techniques before being brought together in the same device.
Another approach is described in United States Patent Application 2001/0053535. This application discloses a microscale biosensor for use in the detection of target biological substances including molecules and cells. The biosensor is a microfluidic system with integrated electronics, inlet-outlet ports and interface schemes, high sensitivity detection of pathogen specificity, and allows processing of biological materials at semiconductor interfaces. In particular, the biosensor has a detection chamber, which is a small well or cavity produced by microfabrication techniques in a wafer and provided with sensing elements such as electrodes for sensing a change in an electrical characteristic or parameter (such as resistance or phase) in the chamber owing to the presence of the target microbiological species. To perform sensing, United States Patent Application 2001/0053535 relies on the integrated metal electrodes which operate based on the principle of impedance measurement. To fabricate the sensor, United States Patent Application 2001/0053535 utilizes top-side processing for the formation of the integrated metal electrodes. The fluidic channels are fabricated in Si using anisotropic potassium hydroxide (KOH) etching. The sensor is said to be effective in detecting microbiological material in the form of pathogenic strain of bacteria approximately 2 micrometers in dimension.
Fabricating the prior art devices as mentioned above is rather cumbersome and expensive. Therefore, an objective of the present invention is to provide an alternative sensor that advantageously avoids or reduces some of the above-mentioned drawbacks of prior art devices in an easy and economical manner.